The present invention relates to a resin-bonded rare earth magnet having good dimensional accuracy and high magnetic properties, particularly to a resin-bonded rare earth magnet in a thin and/or long shape. The present invention also relates to magnet powder-resin compound particles suitable for producing thin and/or long, resin-bonded rare earth magnets and a method for producing such magnet powder-resin compound particles.
Magnet powder widely used for resin-bonded rare earth magnets is generally isotropic magnet powder based on a main phase of an Nd2Fe14B-type intermetallic compound, which is produced by rapidly quenching an alloy melt having a composition comprising an Nd2Fe14B-type intermetallic compound as a main phase to form an amorphous alloy, and after pulverization, if necessary, subjecting the amorphous alloy to a heat treatment to crystallize the Nd2Fe14B-type intermetallic compound. In addition, an alloy having the above composition may be melted and cast by a strip casting method, a high-frequency melting method, etc., pulverized, and then subjected to hydrogenation, phase decomposition, dehydrogenation and recrystallization treatment (see Japanese Patent 1,947,332), thereby providing anisotropic magnet powder having a fine recrystallized structure for resin-bonded magnets. This magnet powder has an Nd2Fe14B-type intermetallic compound as a main phase. The anisotropic magnet powder having a fine recrystallized structure based on an Nd2Fe14B-type intermetallic compound may also be produced by pressing the above thin, amorphous alloy ribbons or flakes at high temperatures by a hot press, etc., and subjecting the resultant thin alloy compact to plastic working such as upsetting, etc.
Recently, resin-bonded rare earth magnets have been required to be as thin as possible with high magnetic properties and dimensional accuracy. When used for electronic buzzers of mobile telecommunications equipment, for instance, gaps between the magnets and vibration plates are controlled to adjust tone quality. Because their assembling is performed in automated lines, it is necessary to improve the dimensional accuracy of electronic buzzers including resin-bonded rare earth magnets for achieving higher performance. Also, high magnetic properties, smaller thickness and strict dimensional accuracy are required for the resin-bonded rare earth magnets for use in spindle motors in hard-disk drives in computers and motors in CD-ROM drives, and further DVD (digital video disk) drives, etc. in the future. Further, integral, long, resin-bonded rare earth magnets are needed, because they make bonding by adhesives unnecessary, thereby eliminating bonding lines and thus reducing the number of assembling steps while improving magnetic properties. Integral, thin, long, resin-bonded rare earth magnets are also demanded.
The term “long” used herein means 10 mm or more in length, and the term “thin” used herein means 3 mm or less in thickness. Thus, it is recently demanded that resin-bonded rare earth magnets be as thin as and/or as long as possible while increasing magnetic properties and dimensional accuracy.
The magnetic properties and dimensional accuracy of thin and/or long, resin-bonded rare earth magnets are largely affected by forming methods and the shapes of magnet powder-resin compound particles. The forming methods of the resin-bonded rare earth magnets include a compression molding method, an injection molding method, an extrusion molding method, etc.
In the case of the compression molding method, magnet powder-resin compound particles for resin-bonded rare earth magnets are charged into a cavity of a molding die and compressed under pressure. Thereafter, heat curing is carried out to produce the resin-bonded rare earth magnets with high mechanical strength and dimensional accuracy. Recent development of compression molding technology such as mechanical pressing and rotary pressing has realized high-speed molding. However, as the resin-bonded magnets become thinner and/or longer, it becomes difficult to charge magnet powder into a die cavity, and it becomes insufficient to exert compression pressure particularly in a depth direction (compression direction). As a result, the resultant resin-bonded magnets have such an uneven density distribution that end portions to which compression pressure is directly applied have a higher density, while a center portion has a lower density. This uneven density distribution leads to uneven magnetic properties and dimensional accuracy among the products.
The injection molding method is advantageous in that it can easily provide moldings formed thereby with various shapes, though the moldings have relatively uneven density distributions like those produced by compression molding. Molding tact is important in the injection molding method, and the above-described progress of pressing technology has deprived the injection molding method of what is conventionally considered advantages, namely high molding efficiency that produces many moldings at the same time. Because magnet powder-resin compound particles are required to have good moldability (flowability), they have to contain high percentages of binder resins. Thus, resin-bonded rare earth magnets formed by the injection molding method have lower magnetic properties than those formed by the compression molding method or the extrusion molding method.
When the extrusion molding method is used, the percentages of magnet powder in the magnet powder-resin compound particles are higher than those produced by the injection molding method, but lower than those produced by the compression molding method. Accordingly, the resin-bonded rare earth magnets formed by the extrusion molding method have magnetic properties between those of the injection molding method and those of the compression molding method. Though the extrusion molding method is suitable for producing long moldings, such moldings have relatively uneven density distributions like those formed by the compression molding method.
The blending of rare earth magnet powder with a binder resin (corresponding to pre-blending in the present invention) has conventionally been carried out by a double-screw extruder, etc., followed by pelletizing to produce magnet powder-resin compound pellets. The conventional magnet powder-resin compound pellets contain considerable pores and are in a ragged irregular shape showing poor flowability (moldability). When such conventional magnet powder-resin compound pellets are subjected to compression molding, the resultant thin and/or long, resin-bonded rare earth magnets have large unevenness in their density distribution, posing the problems that the density is higher in both ends portions to which a compression pressure is applied than in a center portion. In the case of solid-cylindrical, resin-bonded rare earth magnets, their outer diameters have poor circularity. Also, in the case of ring-shaped, resin-bonded rare earth magnets, their outer and inner diameters have poor circularity. When the ring-shaped, resin-bonded rare earth magnets having poor circularity are used for rotors, the rotors have large eccentricity, resulting in large unevenness in gaps between the rotors and the stators. Also, to prevent the rotors from being brought into contact with the stators, the air gaps should be designed taking into consideration the eccentricity of the rotors. This makes it difficult to construct high-efficiency motors.